The present invention relates to a frequency synthesizer applicable to radio communication equipment.
A prior art frequency synthesizer of the kind described includes a variable frequency divider for dividing an output frequency of a voltage controlled oscillator to a predetermined frequency, a fixed frequency divider for dividing an output frequency of a reference oscillator, and a phase comparator for comparing the phases of outputs of the two frequency dividers to produce a voltage which is representative of a difference between the two phases. The output of the phase comparator is applied to the voltage controlled oscillator to control the oscillation frequency thereof, thereby producing a desired frequency. A power source voltage is constantly applied to the voltage controlled oscillator and intermittently applied to the two frequency dividers, the phase comparator and the reference oscillator. A problem is that the prior art synthesizer needs a long lock-up time because of the intermittent power source voltage supply. Another problem is that substantial power is consumed even in a battery saving condition because a power source voltage is constantly applied to the voltage controlled oscillator.